An imaging device that images a front area of a vehicle by a part of an image sensor and images extraneous substance(s) such as rain drop adhered to a windshield of the vehicle by other portion of the image sensor is conventionally known.
A conventional imaging device is disclosed in, for example, Patent Literature 1. The imaging device images a front area of a vehicle where a white line or preceding vehicle on a road surface exists by a first light-receiving portion that occupies an upper portion of about ⅔ of an image sensor and images rain drop adhered onto an outer wall surface of a windshield of the vehicle by a second light-receiving portion that occupies a lower portion of about ⅓ of the image sensor. The imaging device includes a light emitter that emits illumination light and a reflection polarization prism (an illumination light guiding member) which is provided on an inner wall surface of the windshield. The reflection polarization prism transmits the illumination light and is configured to be incident the illumination light on the windshield and to receive reflection light on the reflection polarization prism with the second light-receiving portion of the image sensor. By using the reflection polarization prism, it is possible to be made incident the illumination light on the inner wall surface of the wind shield to satisfy a condition in which the illumination light is totally reflected on an area (non-adhesion area) where the rain drop is not adhered on the outer wall surface of the windshield.
The illumination light reflected on the non-adhesion area of the outer wall surface of the windshield enters the reflection polarization prism from the inner wall surface of the windshield and is emitted from an exit surface of the reflection polarization prism and received on the second light-receiving portion of the image sensor. On the other hand, in an area (rain drop adhesion area) where the rain drop is adhered on the outer wall surface of the windshield, the illumination light passes through the outer wall surface to an exterior of the vehicle. Accordingly, the illumination light reflected on the rain drop adhesion area of the outer wall surface of the windshield is not received on the second light-receiving portion of the image sensor. As a result, it is possible to obtain an image in which an image portion projecting the non-adhesion area has a high brightness and an image portion projecting the rain drop adhesion area has a low brightness, from image data corresponding to the second light-receiving portion of the image sensor. By a difference (contrast) between the images, the rain drop on the windshield is configured to be capable of detecting.